1. Field of the Invention
The present invention relates to a driving device and a driving method for supplying power to an LED.
2. Description of the Related Art
FIG. 1 is a circuit diagram of a device for driving an LED in accordance with the related art.
Referring to FIG. 1, a device 100 for driving an LED includes a power supply Vs, a current switching circuit 10, a voltage detector 20, and a rectifying circuit 30.
The power supply Vs is an AC power supply having a predetermined frequency and the rectifying circuit 30 rectifies AC that is output from the power supply Vs. The current switching circuit 10 includes a plurality of light emitting diodes (hereinafter, referred to as LED; D1 to Dk, k is a natural number) that are connected in series and a plurality of current sources I1 to Ik that determine a path of current flowing from output terminals of each LED. The voltage detector 20 monitors a voltage level output from the rectifying circuit 30 and operates one selected from a plurality of current sources I1 to Ik according to the monitored voltage level.
When the voltage output from the rectifying circuit 30 has a voltage level enough to turn on a first LED D1, the voltage detector 20 selects and operates only the first current source I1. In this case, current ILED1 passing through the power supply Vs and the rectifying circuit 30 flows into a ground GND via the first LED D1 and the first current source I1.
Next, when the voltage output from the rectifying circuit 30 rises to a voltage level enough to turn on both of the first LED D1 and the second LED D2, the voltage detector stops an operation of the first current source I1 and selects and operates only the second current source I2. In this case, the current ILED1 passing through the power supply Vs and the rectifying circuit 30 flows into the ground GND via the first LED D1, the second LED D2 and the second current source I2.
Next, when the voltage output from the rectifying circuit 30 rises to voltage level enough to turn on all of the plurality of LEDs D1 to Dk, only the k-th current source Ik is normally operated and an operation of the remaining current sources I1 to I(k−1) stops. In this case, the current ILED1 passing through the power supply Vs and the rectifying circuit 30 flows into the ground GND via the first LED D1, the second LED D2 to the k-th LED Dk, and the k-th current source Ik.
On the contrary, when the voltage output from the rectifying circuit 30 is decreased, the current source and the LED are turned off in an opposite direction to the foregoing direction.
In case of the LED driving circuit illustrated in FIG. 1 in accordance with the related art, the voltage detector 20 directly detecting the voltage level output from the rectifying circuit 30 is needed, wherein the voltage detector 20 may be implemented by various schemes, but in all cases, may have a problem in that the area and power consumed by the driving device 100 may be large enough not to be able to be ignored.
FIG. 2 illustrates operation characteristics of a device for driving an LED in accordance with the related art.
Referring to FIG. 2, when voltage Vac supplied to the LED strings D1 to Dk configuring the current switching circuit 10 is increased or decreased in a parabola form due to the rectification of power Vs, voltage VLED1 dropping at the LED strings D1 to Dk configuring the current switching circuit 10 is also increased or decreased according to a change in voltage level of the rectified voltage Vac.
However, even though the voltage Vac supplied to the LED strings D1 to Dk configuring the current switching circuit 10 is increased or decreased in a parabola form due to the rectification of power Vs, a magnitude in the current flowing into the LED strings D1 to Dk configuring the current switching circuit 10 is limited. The reason is that an electrical characteristic, that is, a current amount of the plurality of current sources I1 to Ik configuring the device 100 for driving an LED in accordance with the related art is the same.
For this reason, there is a problem in that a power factor of the device for driving an LED in accordance with the related art is low.
As illustrated in FIG. 2, 1Vth means threshold voltage that may turn on the first LED D1 and 2Vth means threshold voltage that may turn on both of the first LED D1 and the second LED D2. Therefore, 5Vth means threshold voltage that may turn on all of the five LEDs that are connected in series.
FIG. 2 illustrates operation characteristics of a case in which all of the five LEDs are connected in series, for convenience of explanation.